Photolithography involves the controlled exposure of a photoresist layer on a substrate to provide a desired design on the substrate. In photolithography, a photomask, which can be a positive or negative of the desired design, is first aligned with the substrate. A photoactive material on the substrate is then exposed using a suitable source of radiation, e.g., visible light or UV radiation.
Photolithography is widely used in the fabrication of integrated circuits. Because the fabrication of integrated circuits typically involves the sequential use of more than one photomask, the proper alignment of the photomask with the wafer and the formation of the designed image on the wafer becomes particularly critical. One device that has been used to align and image photomasks is a stepper. The stepper has found widespread use in light of recent advances in the field of integrated circuits that have resulted in a significant decrease in the size of design features.
In the fabrication of integrated circuits, the linewidth of etched design features are measured and monitored to detect process irregularities that can ultimately lead to degraded circuit performance.
It has been discovered that in the field of photolithography, and in particular, those photolithographic processes that employ a stepper to align and image the reticle (or photomask) onto the wafer, a number of factors associated with the exposure of the wafer can introduce linewidth variation. For example, the devices employed, such as the stepper, the reticle and even the stepper lens, can introduce variations in the linewidth of design elements exposed on a wafer.
Linewidth variation can also be introduced by the process characteristics of other steps in the photolithographic process and in particular, those steps involving the application of the photoresist. Examples of such process characteristics include the photoresist reflectivity, photoresist thickness, as well as the characteristics of the process used to apply and/or develop the photoresist.
One attempt to effectively control linewidth variation has involved the use of linewidth control features (LCF), which are in effect test patterns, introduced onto a wafer. A variety of linewidth control features are known in the art. See, for example, U.S. Pat. No. 5,780,316, which is incorporated by reference in its entirety for all purposes.
During a typical integrated circuit manufacturing process, transistors known as gates are being formed within individual integrated circuits. LCF's, which are additional features having the same width and material composition as the gates, are formed on the individual integrated circuits together with the gates. The linewidth control features may be subsequently examined and measured by, for example, scanning electron microscopy (SEM), to determine whether the gate forming process, in particular the gate width, has been performed properly. If, upon examination, the linewidth control features appear to have the proper width, it is inferred that the gates of individual integrated circuits (which were formed at the same time and by the same process as the linewidth control feature) have been properly formed. Alternatively, the linewidth control features can be electrically tested so as to detect irregularities in linewidth. This allows for the determination of electrical performance which is not directly obtainable from features in the integrated circuit.
However, it has now been discovered that the typical way in which linewidth control features are introduced on the wafer introduces its own set of limitations and/or difficulties. One particular problem relates to the fact that printed fields can vary greatly. For example, each code used in forming an integrated circuit can employ different field and chip sizes as well as different grid locations. Because of this, the location of the linewidth control features within the stepper lens field will vary from one code to another. The LCF's associated with these codes are, therefore, exposed using different portions of the lens field. Accordingly, the linewidth characteristics of the linewidth control features themselves can vary from one code to another due to the variation across the stepper lens field.
This problem, as discovered by the inventor, can be illustrated, e.g., by FIGS. 1a-c. FIG. 1a illustrates that even well manufactured lens can have minor variations that can lead to linewidth discrepancies of 10% or more. For example, the LCF used in connection with Code A (FIG. 1b), because of its placement near the "edge" of the lens will differ in linewidth from the LCF of Code B (FIG. 1c) which is generally exposed using the "mid-section" of the lens. This is true even if the LCF's for Code A and B have the same size and shape on the reticle.
Lens-induced variation not only impacts on the linewidth of the feature itself, but also can adversely impact subsequent exposure steps. That is, because the linewidth control feature is used to optimize exposure energy, the variation in the linewidth control feature caused by the lens field can result in erroneous adjustments in exposure energy. This, in turn, will introduce variations into the linewidth across a wafer.
This problem, which has only been discovered by the inventor, necessitates an improved process for linewidth control, particularly in a photolithography stepper environment used in fabricating integrated circuits.